Fiber optic communications links carry information signals on a laser-generated lightwave carrier. Most fiber optic communications systems until recently have employed direct modulation techniques to mix tile lightwave carrier and the information signals. The lightwave carrier is typically produced by a semiconductor laser biased at a point above that required to cause the device to emit laser light. The direct modulation technique uses the information signal, commonly though not necessarily a radio frequency signal, injected at the bias point of the semiconductor laser to vary the current that drives the laser, thus modulating the intensity of the laser signal. The output of the laser varies with the fluctuations of the information signal. These fluctuations are detected by a photodetector device at the receiver to reproduce the information signal.
However, there are disadvantages associated with direct laser modulation. The laser lightwave carrier is amplitude modulated by the information signal, producing a double-sideband signal with a large carrier component. The large amount of transmitted power in the carrier component produces a large DC output at the detector, limiting the dynamic range of the link. The laser, being a non-linear semiconductor device, can "chirp," emitting a sudden amplitude spike or trough impulse when the bias current is rapidly varied about its selected bias point. Direct modulation also increases noise in the signal and produces significant high order distortion products.
The advent of electro-optical modulators has permitted system designers to employ fixed-bias, constant output laser sources to generate the lightwave carrier. The constant lightwave carrier and the information signals are separately supplied to the electro-optical modulator. The modulator impresses the information signal on the carrier by using the information signal to electro-optically affect the passage of the lightwave carrier through a waveguide.
The modulated signal produced by electro-optical modulation is generally superior to that resulting from direct laser modulation. It is not subject to "chirp" because the laser bias point is constant. High order distortion is less pronounced. System noise is reduced. The dynamic range limitation of amplitude modulated signals remains, however, a result of the carrier component in the modulated signal.
In order to increase the dynamic range of the system, the carrier component inherent in the amplitude modulation of the lightwave carrier must be removed. Such systems are referred to as suppressed-carrier systems. With the carrier removed, substantially all the transmitted signal power is located in the information-bearing upper and lower sidebands. The DC component in the modulation product is removed, extending the potential dynamic range of the link.
The price of removing the carrier component before transmission is the necessity of providing a laser frequency standard at the receiver to demodulate the signal. Usually the carrier component is reintroduced at the receiver by means of a local laser operating at the same wavelength (frequency) as the source.
However, the necessity of providing a local laser standard at the receiver can be eliminated by transmitting the carrier along with, but separate from the transmitted suppressed-carrier modulated signal. This must be accomplished without interaction or mixing between the carrier and the modulated signal. The simultaneous transmission of the suppressed carrier modulated signal and its carrier without interaction is possible if these two signals have different optical polarizations.